Strain gages, which are an important diagnostic tool, generally convert mechanical motion into an electrical signal. The strain that such a gage undergoes varies proportionally with the change in capacitance, inductance, or resistance, but the change in resistance is the characteristic most widely used as a measure of strain. Ideally, a strain gage resistance changes only when a surface to which the gage is attached deforms. In reality, various factors such as temperature, the nature of materials, the adhesive that bonds the gage to the material surface, and the stability of the metal can alter the measured resistance. Thus, for example, in choosing the type of strain gage material to use, one has to consider material characteristics such as temperature sensitivity, stability, and resistance.
Various types of strain gages are known. Mechanical gages suffer from low resolution and bulkiness, and they are not easy to use. Typical optical gages are more sensitive and accurate, but they are relatively fragile. For example, while a photoelectric gage can be relatively small, photoelectric gages are expensive and fragile.
Sensors can be used to monitor environmental conditions such as pressure or temperature. For example, such conditions could be monitored using sensors mounted inside an automobile engine or one or more of the engine's components.
Sensors can be particularly useful in monitoring or optimizing engine performance by using them to monitor conditions inside one or more cylinders of an engine. A cylinder head functions to seal the top of the cylinders, provide a mounting for valve train components, provide guides and ports for the intake and exhaust valves, and provide the spark plugs access to the combustion chamber. Each recessed area above the piston in the cylinder head is part of a combustion chamber in which the air and fuel mixture burns. The intake of air and fuel mixture and the exit of burned gases must be regulated to allow the engine to operate properly.
The cylinder head must be sealed tightly to contain the high pressure resulting from the air and fuel combustion. The cylinder head gasket, which is placed between the cylinder head and the engine block, performs this function. Because of the severe environment in the cylinder head, the components around the cylinder head must be able to withstand severe conditions such as temperatures that may vary from −40° C. to +250° C. The cylinder head gasket may also be subjected to very high pressures. Thus, it would be advantageous to be able to monitor in-cylinder conditions using sensors that could withstand the harsh environment within or near the vicinity of the cylinders or other engine components. Monitoring in-cylinder pressure is desirable to obtain better control over engine operation and performance. However, conventional approaches require expensive hardware such as pressure sensors located at each combustion chamber on the cylinder head region.
In addition, traditional wired sensors are usually difficult to assemble and less reliable due to the number of wires involved. Signals passing through wires often cause cross talking and interference with the signal transmission in adjacent wires. Signals passing through wires also cause electromagnetic interference in adjacent wires unless some type of shielding is used. Thus, the signals may become distorted or skewed because of these factors.